Due to self-emission and no dependency on viewing angle, as well as capability of plane emission and reduction in thickness, application of an organic EL device to a display of a TV or a mobile phone, an illumination or the like has been studied.
An organic EL device has a configuration in which a thin film of an organic compound is sandwiched between a cathode and an anode. The methods for forming a thin film are roughly divided into the deposition method and the coating method. The deposition method is a method in which a low-molecular weight compound is mainly used, and a thin film is formed on the substrate in vacuum. The industrial-base-production thereof proceeds. On the other hand, the coating method is a technique in which a thin film is formed on the substrate by using a solution (e.g. inkjet and printing). The deposition method has a high material utilization efficiency, and hence is suited to an increase in area and an enhancement of resolution. This method is a technology which is deemed to be indispensable in an organic EL large-sized display in the future.
The vacuum deposition method using a low-molecular material has an extremely low utilization efficiency of the material. In addition, if the size is increased, the degree of distortion of a shadow mask becomes large. Therefore, deposition on a large-sized substrate is difficult, and as a result, fabrication of a display using a large-sized organic EL substrate is difficult. In addition, the production cost also increases.
On the other hand, in the case of a high-molecular weight material, it is possible to form a homogenous film by applying a solution obtained by dissolving the high-molecular weight material in an organic solvent. A coating method represented by the inkjet method or the printing method can be used by utilizing such a nature of a high-molecular weight material. Therefore, it is possible to enhance the material utilization efficiency to near to 100%. In addition, since it can be applied to substrates of various shapes ranging from a small-sized substrate to a large-sized substrate, the production cost of the device can be significantly reduced.
However, in general, the coating method is not suited to a stacked-type device, and has a problem that it is not easy to improve the performance of the device.
The reason for unsuitability of applying the coating method to a stacked-type device is that, when stacking layers, films which have been formed in advance are inevitably dissolved. Specifically, in a stacked-type organic EL device fabricated by the coating method, a hole-injecting layer and a hole-transporting layer have to be insoluble in a solvent used when an emitting layer is formed.
Therefore, at present, most of organic EL devices obtained by the coating method are limited to a two-layer structure in which a hole-injecting layer is formed by a water-dispersion liquid of polythiophene:polystyrene sulfonic acid (PEDOT:PSS), and an emitting layer is formed by using an aromatic organic solvent such as toluene. Since the PEDOT:PSS layer is not dissolved in toluene, it is possible to fabricate such two-layer structure.
A hole-transporting material is formed into a hole-transporting layer and stacking on a hole-injecting layer by using an organic solvent such as toluene, xylene, dioxane and cyclohexanone. In this case, the adhesiveness to a hole-injecting layer emerges as a problem. If a hole transporting material has a low adhesion to a hole-injecting layer, current leakage may occur. In addition, since the surface flatness of a resulting layer is not good and the interface between the layer and an emitting layer is not flat and smooth, the life of a device obtained may be shortened.
As for a hole-transporting layer, use of a high-molecular weight compound has been studied in expectation of improved physical or thermal durability. For example, Patent Document 1 reports an organic EL device using as a polymer polyvinyl carbazole (PVCz) having a hole injecting ability. Since the glass transition temperature (Tg) of PVCz is extremely high, the resulting thin film has an excellent stability, whereby an organic EL device having a high durability can be fabricated.
However, the device thus obtained has problems that it requires a significantly high driving voltage, and has insufficient luminous efficiency and short lifetime.
Further, Patent Document 2 discloses an organic EL device using a polymer obtained by co-polymerizing a vinyl anthracene derivative and a vinyl carbazole derivative. However, the device has problems that the luminous efficiency is poor and the lifetime is short.
On the other hand, various studies have been made on a high-molecular weight electroluminescence material since it can be formed into a film by applying the solution thereof and printing (Patent Documents 3 to 7).
However, it has problems that an organic EL device using the above mentioned high-molecular weight compound does not always have sufficient device properties such as a long life (half life) and a high luminous efficiency.
Further, as a material for a hole-transporting layer or a material for an emitting layer, many high-molecular weight materials using a carbazole unit have been studied. However, in the both cases of a material for a hole-transporting layer (Patent Documents 8 and 9) and a material for an emitting layer (Patent Documents 10 to 13), since high-molecular weight materials are synthesized by random co-polymerization, the resulting device has low reproducibility. In addition, it has problems that partial structural defects occur due to an irregular main chain skeleton and trap of holes occurs, thereby to lower the device performances (efficiency, lifetime).
Moreover, the materials for a hole-transporting layer disclosed in Patent Documents 8 and 9 have a problem that the resulting devices have deteriorated performances (efficiency, lifetime) due to poor adhesion to a hole-injecting layer or insufficient surface flatness of a hole-transporting layer obtained.